Consolidation (weeks 10-19): Patients receive oral mercaptopurine on days 64-119; methotrexate IV over 24 hours and leucovorin calcium IV, or orally as tolerated, on days 71, 85, 99, and 113; and methotrexate IT on days 71, 85, 99, and 113. (Patients begin methotrexate once blood counts recover and only if bone marrow is M1).